PILE FOUNDATIONS

A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground to act as a steady support for structures built on top of it.

Pile foundations are used in the following situations:

When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer.

When a building has very heavy, concentrated loads, such as in a high rise structure, bridge, or water tank.

There are two fundamental types of pile foundations (based on structural behaviour), each of which works in its own way.

End Bearing PilesIn end bearing piles, the bottom end of the pile rests on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts like a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load therefore bypasses the weak layer and is safely transferred to the strong layer.

Friction Piles

Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil. To visualise how this works, imagine you are pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream. Once you have pushed it in, it is strong enough to support some load. The greater the embedment depth in the ice cream, the more load it can support. This is very similar to how a friction pile works. In a friction pile, the amount of load a pile can support is directly proportionate to its length.

what are piles made of?

Piles can be made of wood, concrete, or steel. In traditional construction, wooden piles were used to support buildings in areas with weak soil. Wood piles are still used to make jetties. For this one needs trees with exceptionally straight trunks. The pile length is limited to the length of a single tree, about 20m, since one cannot join together two tree trunks. The entire city of Venice in Italy is famous for being built on wooden piles over the sea water.

Cross sections of various pile foundations

Concrete piles are precast, that is, made at ground level, and then driven into the ground by hammering - more on that later. Steel H-piles can also be driven into the ground. These can take very heavy loads, and save time during construction, as the pile casting process is eliminated. No protective coating is given to the steel, as during driving, this would be scraped away by the soil. In areas with corrosive soil, concrete piles should be used.

how piles are used

As pile foundations carry a lot of load, they must be designed very carefully. A good engineer will study the soil the piles are placed in to ensure that the soil is not overloaded beyond its bearing capacity.Every pile has a zone of influence on the soil around it. Care must be taken to space the piles far enough apart so that loads are distributed evenly over the entire bulb of soil that carries them, and not concentrated into a few areas.

The load pattern of the piles on the soil surrounding them. This is also called a zone of influence.

Engineers will usually group a few piles together, and top them with a pile cap. A pile cap is a very thick cap of concrete that extends over a small group of piles, and serves as a base on which a column can be constructed. The load of this column is then distributed to all the piles in the group.

how piles are constructed

Piles can be either cast-in-place or precast driven piles.

Cast-in-place piles are made in the following steps:

hammer a thin-walled steel tube into the ground

remove all earth left inside the tube

lower a steel reinforcement cage into the tube

cast the pile by pouring wet concrete into the tube

The thin walled steel tube is called the casing, and only serves to form a secure mould for casting concrete that is free from earth and debris. It has no structural role to play after the casting is complete.

Some soils are highly cohesive, meaning that if one drills a hole into the soil that is say 1 foot wide by 50 feet deep, then the soil holds the shape of the hole and does not collapse into the hole and block it. If such soil is present at the site, then one does not need to leave a casing in place: one can use the casing to drill the hole for the pile, and then remove it, and then cast the pile in place. This saves costs as the same casing tube can be used to drill holes for all the piles.

Precast Driven Piles are first cast at ground level and then hammered or driveninto the ground using a pile driver. This is a machine that holds the pile perfectly vertical, and then hammers it into the ground blow by blow. Each blow is is struck by lifting a heavy weight and dropping it on the top of the pile - the pile is temporarily covered with a steel cap to prevent it from disintegrating. The pile driver thus performs two functions - first, it acts as a crane, and lifts the pile from a horizontal position on the ground and rotates it into the correct vertical position, and second, it hammers the pile down into the ground.

Piles should be hammered into the ground till refusal, at which point they cannot be driven any further into the soil.

Special Piles

Pile driving is very noisy and causes massive vibrations through the soil. For this reason, it is sometimes difficult to use them in sensitive locations. For example, if an operational hospital or science lab is to be extended, driving piles would cause unwanted disturbance. Their use is also restricted in residential areas in many countries. The vibrations could also cause structural damage to older buildings that are close by. In such situations it is possible to use micropiling or helical piling, neither of which rely on hammering.Micropiles or minipiles are small piles that are constructed in the following way:Step 1: a hole a little larger than the pile diameter and the full length of the pile is dug into the ground using an apparatus like a soil boring machine.Step 2: a precast concrete pile is lowered or pushed into the hole.Step 3: a concrete grout is poured into the gap between the pile and the earth.Helical piles are steel tubes that have helical (spiral) blades attached to them. These can be drilled into the ground, meaning that the pile acts as a giant drill bit, and is rotated and pushed into the ground from above, much like a screw drills into wood. Once the steel pile is driven into the ground, a pile cap is poured on top of the pile to prepare it for the construction above.